Precision fluid flow control devices, such as fuel supply units for aerospace systems and oxygen/air metering units employed in hospitals, typically incorporate some form of solenoid-operated valve through which a desired rectilinear control of fluid (in response to an input control current) is effected. In addition to the requirement that fluid flow be substantially linearly proportional to applied current, it is also desired that hysteresis in the flow rate versus control current characteristic (which creates an undesirable dead band in the operation of the valve) be maintained within some minimum value.
For this purpose, one customary practice has been to physically support the solenoid's moveable armature within its surrounding drive coil by means of low friction bearings, such as Teflon rings. However, even with the use of such a material, the dead band is still not insignificant (e.g. on the order of 45 milliamps), which limits the degree of operational precision of the valve and thereby its application.
One proposal to deal with this physical contact-created hysteresis problem is to remove the armature support mechanism from within the excitation coil (where the unwanted friction of the armature support bearings would be encountered) to an end portion of the coil, and to mount the armature to a spring mechanism that is effectively supported outside of the coil. An example of such a valve configuration is found in the U.S. Pat. to Everett, No. 4,463,332,issued July 31, 1984. In accordance with the patented design, the valve is attached to one end of an armature assembly supported for axial movement within a cylindrical housing that contains an electromagnetic coil and a permanent ring magnet surrounding the coil. One end of the solenoid contains a ring and spring armature assembly, which is located substantially outside the (high flux density) bore of the excitation coil and the position of which can be changed to adjust the flux gap in the magnetic circuit and thereby the force applied to the valve. Disadvantageously, however, this shifting of the moveable armature to a location substantially outside of the high flux density of the excitation coil, so as to reduce the friction-based hysteresis problem, creates the need for a magnetic flux booster component, supplied in the patented design in the form of a permanent magnet. Thus, although the intended functionality of such a structure is to adjust magnetic permeance and maintain linearity in the operation of the valve to which the armature is attached, the designs of both the overall solenoid structure and individual parts of which the solenoid is configured, particularly the ring spring armature assembly (which itself is a complicated brazed part) and the use of a permanent magnet, are complex and not easily manufacturable using low cost machining and assembly techniques, thereby resulting in a high pricetag per unit.